We propose to study the enzymatic removal from DNA of 7-methylguanine residues in which the imidazole rings have been opened. This compound, 2,6-diamino-4-hydroxy-5-N-methylformamido-pyrimidine, is recognized by a partially purified fraction from E. coli which we have obtained. X-irradiated DNA will be tested with this fraction to determine if the enzyme recognizes similar bases and if so, the enzyme, a glycosylase, will be used to try to characterize those bases. Testing for this activity in mammalian cells will also be done. A second project involves the ability of radiosensitive and radioresistant lines of the murine lymphoma strain L-5178y to remove bases from DNA alkylated with MNU.